In a digital communication system, more particularly, in a mobile communication system, looking for a high modulation method with excellent performance under a given channel condition is always an important task. For a radio frequency unit of a digital intermediate frequency transmitter, when modulation signals are maintained on a relatively constant voltage level, a radio frequency amplifier generally operates with the best efficiency. However, large power peaks of the modulation signals often cause the operation of a radio frequency amplifier with lower efficiency and worse linearity. Therefore, it is desired that in a radio frequency amplifier, the ratio of peak power to average power of modulation signals approximates to as 1 or Decibel of 0 db as possible. A digital intermediate frequency transmitter of a multi-carrier system is shown in FIG. 1, wherein digital multi-carrier signals are modulated by a baseband unit. After passing through an up-converter, the multi-carrier signals are inputted into a radio frequency unit via a DAC (digital-to-analog converter). If the ratio of peak power to average power of the multi-carrier signals is not limited requisitely by the multi-carrier system, the maximum power of the radio frequency amplifier in the radio frequency unit will be significantly larger than the average power in order to ensure no signal distortion and in order to prevent the spectrum spreading from occurring. This causes not only wasting of power at a radio frequency unit but also significant trouble in designing a radio frequency unit.
Currently, in a multi-carrier modulation method, several carriers are modulated in parallel by using several data streams. Combined signals of multi-carrier modulation of multi-user are formed at base stations. A digital intermediate transmitter is typically shared by the base stations, rather than having one transmitter for one carrier, to reduce cost.
However, the biggest disadvantage of the multi-carrier modulation method is that the combined signals have very high ratio of peak power to average power, that is, the maximum power of a radio frequency amplifier in a radio frequency unit is significantly larger than the average power. Typically, to prevent distortion and reduce the spectrum spreading, a large linear dynamic range of a radio frequency amplifier is required for amplifying the combined signals, which force the designers to use an amplifier with a large linear range, whereby the peak power is significantly larger than the average power of the combined signals. This significantly increases the cost of the multi-carrier system. Thus, under the premise of not decreasing the other performance specifications, a ratio of peak power to average power of the combined signals should be decreased as much as possible, and making the signals as having constant envelope or approximately constant envelope as possible is an objective of the designers.
An out-of-band compensation method has been proposed to improve the ratio of peak power to average power of the signals. The out-of-band compensation method is a method of producing a certain out-of-band compensation signal based on input signals, and adding the original signals with the compensation signal to obtain a combination signal that has an approximately constant envelope, or to obtain a reduced ratio of peak power to average power of the signals. It is desired that the power of the compensation signal and an error signal rate of the system are as small as possible, and that the combination signal also satisfies the requirements of communication system protocols. Because the compensation signal is added out-of-band, the effect on the original signals is small.
However, one of the disadvantages of the out-of-band compensation method is that when a large amount of compensations are required, the computation of a compensation signal is complicated and tedious. Also, sometimes a real time computation may be impossible. As a result, the ratio of peak power to average power of the signals may not be improved by the out-of-band compensation method. When the compensation is small, the improvement of the ratio of peak power to average power is limited. Thus, the out-of-band compensation method does not function well.
Another proposed method is a probabilistic waveform clipping method. The principle of this waveform clipping method is that when the amplitude of a signal is above a certain threshold, the amplitude of the signal is set to the threshold, and when the amplitude of a signal does not exceed this threshold, the amplitude of the signal does not change. Although reducing the peak power of the signals to a threshold level or below may help satisfy the requirement of the ratio of peak power to average power, the strength of the signals tends to be weak after the waveform clipping process. Also, the in-band noise will be increased which would affect the transmission of the signals accordingly. In order to increase an amplitude threshold of the waveform clipping, it is required that the power be increased significantly whereby the cost, the energy source, and the requirements on the other aspects of the system will also be increased. Thus, the application of the waveform clipping method is not practical.
Accordingly, it is desirable to provide a method and apparatus for reducing the ratio of peak power to average power of multi-carrier signals. The method and apparatus in accordance with the principles of the present invention not only ensure the signal quality, but also reduce the signal noise.